Frequency domain bitmap triggering using color, density and correlation based triggers

ABSTRACT

An RF test and measurement device, including a front end for receiving a time-varying signal and a real-time engine for generating digital frequency domain spectrums based on the time-varying signal. The device also includes a memory subsystem containing a frequency domain bitmap which is updated through sequential receipt and storage of the digital frequency domain spectrums. The real time engine is further configured to monitor the frequency domain bitmap for occurrence of a signal characteristic, and in response to detection of the signal characteristic, cause a capture of the time-varying signal into a storage location of the RF test and measurement device.

CROSS-REFERENCE TO RELATED APPLICATION(S)

This application claims priority from U.S. Provisional Patent Application No. 61/160,209, entitled “Frequency Domain Bitmap Triggering,” filed Mar. 13, 2009, the disclosure of which is hereby incorporated by reference in its entirety and for all purposes.

BACKGROUND

A variety of test and measurement devices exist for use in connection with RF signals. In many cases, these devices include components that provide real-time processing and other functionality that occurs as a time-varying signal is received, and other components that perform off-line functionality, such as a detailed signal analysis that occurs after a signal event has been captured and stored.

Test and measurement operations are often controlled using triggering methods. For example, in the above case of discovering and capturing signal events for subsequent study, signal captures can be based on a trigger that is activated when certain signal characteristics are detected. Use of such a trigger conserves computing resources by only capturing signals that may potentially be of interest.

Although existing triggers are adequate in many cases, a variety of common signal situations exist that cannot be effectively detected by existing triggers. In addition, many existing triggering methods are imprecise, and limit the ability of the user to particularly specify signal events of interest.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic depiction of a test and measurement system according to the present description.

FIGS. 2 and 3 are schematic depictions of exemplary frequency spectrum bitmaps.

FIG. 4 shows an exemplary method for maintaining a frequency spectrum bitmap and performing triggers in response to bitmap data.

FIG. 5 shows an exemplary frequency spectrum bitmap, including two different regions which may incur different waveform prevalence/density.

FIGS. 6-8 show exemplary frequency spectrum bitmaps with bitmap samples correlating variously to a reference signal event.

FIG. 9 is a schematic depiction of various bitmaps that may be employed in connection with the systems and methods of the present description.

FIG. 10 shows a user interface system and method for creating frequency spectrum bitmap triggers.

DETAILED DESCRIPTION

Referring now to FIG. 1, the figure depicts a signal measuring and testing system 20 according to the present description. System 20 includes a front end 22 for receiving a time-varying signal 24 and for optionally performing various processing operations on the incoming signal. Optional processing may include filtering, mixing, down-conversion, etc. As indicated, additional components 26 may be employed to convert the incoming signal into digital form, and to perform decimation operations and/or make phase and amplitude corrections. The depicted configuration is exemplary, and those skilled in the art will appreciate that a variety of other arrangements may be employed.

At node 28 in the signal path, the signal(s) exists as a digitally processed version of time-varying signal 24. This, in turn is applied to real-time engine processing block 30. Among other things, block 30 is configured to repeatedly generate digital frequency domain spectrums 31 based on the time-varying signal 24. Spectrum generation may be performed using any number of digital frequency transform techniques, including chirp-z, FFT and variable-length FFT.

System 20 also includes a memory subsystem 32, which is operatively coupled (e.g., via a bus) with the front-end blocks and the real time block 30. Furthermore, system 20 typically also includes a display subsystem 34 and a processing subsystem 36. As discussed further below, memory subsystem 32 may provide a data store containing a frequency domain bitmap constructed from spectrums 31. System 20 may also include physically or logically separate storage location 38, which may be employed for storage of signals that will be subjected to detailed offline analysis.

Generally, memory subsystem 32 is configured to provide memory space for supporting real-time functionality of system 20. For example, a portion of time-varying signal 24 typically is retained in memory while frequency domain transformation and other real-time operations are performed at block 30. One reason for the retention is to enable a portion of the time-varying signal to be captured and analyzed more extensively offline, for example using processing operations that are more intensive and resource-consumptive than could be performed during real time processing. Setting aside a portion of the time-varying signal in this manner may be referred to as an “analysis capture.” Such a set-aside typically is done in response to detection of a triggering event, as will be explained in more detail below.

By combining real-time processing with selectively-triggered offline processing, measurement and test operations can be made more efficient, and scarce computing resources are used only when needed. In one sense, the real time block 30 operates as a screening tool for making decisions about when a more comprehensive analysis should be performed, and on what specific data it should be performed on. Furthermore, triggering can provide substantial efficiency for users, by eliminating trial and error associated with capturing data of interest, and/or by eliminating the need to capture and sift through large amounts of raw or processed data to find signals of interest.

Turning now to FIG. 2, storage of frequency domain information will be discussed. In particular, in many of the examples herein, frequency domain data is stored in a bitmap structure 40, in which quantized power level information is expressed as a function of frequency. Typically, information from a plurality of the digital frequency domain spectrums 31 will be accumulated into the bitmap data structure. Indeed, in many example embodiments, the bitmap structure 40 is updated and incremented through repeated successive application of the digital frequency domain spectrums as they are generated by block 30.

The bitmap typically is generated using a high transform rate, which is desirable in many settings to facilitate detection of infrequent events. Though desirable for event detection, the high rate will often be too fast for the display device or devices used in connection with subsystem 34 to display the bitmap data, and will normally be well beyond what human eyes can perceive. Accordingly, the incoming transforms are written into the database at full speed and then transferred to the screen at a viewable rate. For example, the display may be driven by sampling the database 10, 20, 30 or 60 times a second. These are but examples—other frame rates may be employed.

As demonstrated by the bitmap figures herein, the bitmap structure may be thought of as a grid created by dividing a spectrum graph into rows representing trace amplitude (power) values and columns for points or ranges on the frequency axis. Each cell in the grid contains a count of how many times it was hit by an incoming spectrum. By tracking these counts, the system can implement proportionality to allow the user to visually distinguish rare transients from normal signals and background noise. The reader will appreciate that the bitmap figures herein provide simplified demonstrative examples, and that the actual embodiment of the database(s) stored in memory subsystem 32 will likely contain hundreds of columns and rows.

Comparison of FIG. 2 and FIG. 3 shows the relationship between a single digital domain frequency transform and the accumulation of many such transforms into the bitmap database. No accumulation has occurred with respect to FIG. 2—it shows a bitmap representation of a single frequency transform, and indicates the power level of the transform for each frequency in the range of interest. More specifically, FIG. 2 shows what the bitmap cells might contain after a single spectrum is mapped into it. Blank cells contain the value zero, meaning that no points from a spectrum have fallen into them yet.

Over time, however, when different samples are taken of the time-varying signal, the individual frequency spectrums will vary and yield different values of frequency and power. The bitmap representation 42 of FIG. 3 shows a bitmap that has been updated (incremented) by applying and storing nine different digital frequency domain transformations in the bitmap. More particularly, the bitmap of FIG. 3 includes the data from the spectrum shown in FIG. 2, but with eight additional spectrums added. The nature of the spectrums applied to the bitmap is that, for each spectrum, some amplitude/power value is indicated for each value along the frequency axis, even if it is a noise floor or other baseline value, or even a value set aside to represent “invalid point”. Indeed, in the example of FIG. 3, one of the spectrums was generated while the incoming time-varying signal was absent, as reflected by the string of “1” values at the noise floor.

In some cases it will be desirable to map occurrence values to a color scale, in order to provide an enhanced visual representation of the data. For example, the following color mapping could be used for the nine-spectrum example described above:

Number of Occurrences Color 0 Black 1 Blue 2 Light blue 3 Cyan 4 Green blue 5 Green 6 Yellow 7 Orange 8 Red orange 9 Red In this example, warmer colors such as red, orange etc. are used to indicate higher numbers of occurrences. This is but an example—a variety of other intensity-grading schemes can be used.

Bitmap hit counts typically are accumulated over time. Because the rate at which spectrums arrive is faster than the display system typically can handle, a “frame” of spectrums (e.g., thousands of spectrums) is accumulated into the bitmap before the next operations are performed on the bitmap. For example, in one embodiment, 48,000 spectrums are applied to the bitmap database each second. Using a frame rate of approximately 33 times per second, the result is that approximately 1400 spectrums are applied to the bitmap database during each display frame. In another embodiment, 292,000 transforms per second are generated, and a 20 Hz frame rate is employed, such that approximately 15,000 spectrums are applied to the bitmap per display frame.

In any case, after a frame has been completed and its count data sent along to the next process, its count values can either be discarded or not. Maintaining the count values from one frame to the next may be referred to as persistence. When persistence is employed, existing counts in the bitmap database are maintained and the power/frequency values of the newly-arriving spectrums are added to increment the counts, rather than simply restarting the counts and replacing them with the values from the most recent frame.

Different types or degrees of persistence may be employed in connection with the storage and display of bitmapped spectrum data. For example, “infinite persistence” refers to maintaining the full accumulated count from each frame to the next, with each succeeding frame adding to the existing counts. On the other hand, “variable persistence” refers to maintaining less than the full count, which can be implemented in various ways, such as by reducing the count by a fixed fraction for each successive display frame.

One situation illustrating persistence and its potential utility is an anomalous signal arising only once during a period of observation. Assume that this signal, although present only once, was present for all 1465 of the spectrum updates in a frame. Assume further that a variable persistence scheme is employed in which the bitmap counts are decremented by 25% each frame. In this example, the affected cells in the bitmap would start out with a value of 1465 and would be displayed full-force when the bitmap database was sampled for display of that initial frame. One frame later, the occurrence values in the affected cells would be decremented by 25% to 1099. Another frame later, they would be decremented to 824, and then smaller and smaller until they become so dim as to be invisible. On the screen, a bright trace would be displayed initially with a spike at the signal frequency. As successive frames are transferred to the display, the part of the displayed spectrum where the signal occurred would diminish (e.g., by fading and/or changing to colors indicating lower density). Meanwhile, the pixels at the noise level would start to brighten below the fading signal. In the end, only a baseline trace would exist at the noise floor.

The spectrum measurement systems and methods described above can provide numerous advantages in a variety of settings. Samples from the time-varying signal are converted into spectrum traces at a high rate so that there is little or no dead time between waveform updates. Furthermore, the spectrum traces are fed to a bitmap database that can be implemented with adjustable parameters to boost the visibility of signal events. As described above, persistence may be implemented in various ways, and the waveform prevalence in different portions of the spectrum may be reflected in the display using various intensity grading and color schemes in connection with the bitmap.

The reader will appreciate that the principles of the present discussion can be extended to accommodate analog frequency domain spectrums. In such a case, the analog spectrums could be sampled and processed so as to produce repeated update of a bitmap data structure. Though the particulars of such a scheme might vary, the general effect would be to provide digitized versions of the analog spectrums. Thus, for purposes of the present discussion, the analog spectrums would yield a plurality of digital frequency domain spectrums (through conversion) that would be repeatedly applied to update the bitmap database.

In many cases it will be desirable not only to observe a particular signal event or events, but also to perform a detailed analysis of the event and/or of neighboring regions of the time-varying signal. A variety of triggering methods exist, including various types of external and internal triggers, including level triggering, power triggers, frequency mask triggers and the like. To analyze transients and other infrequent signal events, it is often useful to employ some form of internal trigger, in which the triggering is performed in response to some characteristic of the signal being observed.

In any case, once a trigger is declared, an analysis capture of the time-varying signal is taken using various acquisition settings, such as capture length, delay, etc. Although the existing triggers are adequate for a fairly wide variety of signals, there is a signal situation that is relative common in digital RF systems for which none of the existing internals triggering methods work. For example, in a given frequency range, signals may occur at a variety of different power levels at different times. A power trigger can catch signals with the highest power, frequency mask triggers can catch signals that exceed some power threshold at a specific frequency, but the “signal under signal” scenario remains elusive.

The bitmap and display persistence described herein may be used to construct trigger mechanisms to overcome the detection problem described above and/or to provide other utility and advantages. Referring now to FIG. 4, an exemplary triggering method will be described.

As shown at 80, the method may first include establishing a region of interest. Typically, this region will be established in association with the bitmap data structure described above. For example, a specific grouping of pixels on the bitmap may be specified as the region of interest. Typically the grouping will be a rectangular region of bitmap cells that are used to drive pixels on the display, though other groupings may be employed.

Generally, some form of user input is received in order to establish the region of interest. For example, a keyboard may be used to specify X and Y ranges of interest corresponding to power and frequency ranges to be observed. Alternatively, graphical methods may be employed, such as receiving a mouse input or other input used to draw a box on a display of the bitmap.

As shown at 82, the method also includes establishing the triggering criteria to be employed. In the examples discussed herein, the trigger is internal and the searched-for characteristics of the signal are specified as or translatable into bitmap data occurring in the region of interest. Specific examples that will be discussed in more detail below include triggers based on intensity grading or color values in the bitmap, and triggers based on signal density and signal correlation.

Steps 86, 88, and 90 relate to the iterative operation of the system that occurs following specification of the region of interest and triggering criteria. Steps 86 and 88 depict the repeated generation of digital frequency domain transforms, and the application of those transforms to update the bitmap database. In one embodiment, the trigger criteria are tested for every transform applied to update the bitmap database. In another possible embodiment, the triggering criteria are assessed only once per display frame, or only once every n transforms, rather than upon every individual transform update to the bitmap database. Accordingly, in the depicted example, the bitmap database is updated N times, where N is the number of transforms per display frame, before processing flow moves on to the testing for satisfaction of the triggering criteria. It should be appreciated, however, that the triggering criteria may be tested on any desired schedule, including at intervals that do not coincide with the display frame rate of the device. That said, it will often be desirable to match the testing to the sampling of the database for display purposes, since the triggering events are often understood by the user in terms of signal phenomena that are visible on the display, either directly or through enhancements provided by persistence, color coding and other features. In other settings, however, time resolution requirements may dictate a preference for applying the test for every transform update to the bitmap.

As seen at 92, after a frame's worth of transforms have been accumulated into the bitmap structure, processing flow moves on to testing of the triggering criteria. Prior to testing of the trigger, however, the accumulated values in the bitmap may be decreased or decayed according to the particular persistence regime being employed, as shown at 90. On the other hand, it will at times be desirable to conduct the trigger test prior to application of any decay or persistence techniques. For example, it may be preferable to perform density triggering on bitmap data prior to adding persistence, or to perform the density test on persisted bitmaps that have been or cleaned to factor out persistence effects.

Continuing with the figure, if a trigger is not declared, processing flow returns to 84, where another frame's worth of transforms are applied to the bitmap prior to the next testing cycles, and so on. On the other hand, as shown at 94, an analysis capture of the time-varying signal is performed if the screening analysis on the region of interest results in a trigger declaration. As previously described, analysis capture typically will involve storage of a desired portion of the time-varying signal in a logically or physically separate location, to allow the captured data to be subjected to a deeper, more comprehensive analysis. As previously described, various acquisition settings may be employed to specify exactly what portion of the time-varying signal is captured when a trigger is declared. Two specifications that are often employed are a specification of the length (in time) of the portion of the time-varying signal that is to be retained for deeper analysis, and an indication of where in time the beginning of the sample should occur, relative to the moment of the trigger event. Furthermore, it will be appreciated that portions of the time-varying signal occurring before and/or after the trigger declaration may be included in the analysis capture that is retained in memory subsystem 32.

Turning now to particular examples of trigger specification, one class of trigger specifications may be constructed using color characteristics of the bitmap. It should be appreciated that when a trigger is specified in terms of color, the triggering will thus be dependent upon the intensity grading or color schemes that are used in connection with displaying samples of the bitmap structure (e.g., on display subsystem 34). The color criteria may be defined in various ways for the specified area of interest. For example, the triggering criteria might be that a certain percentage of the pixels in the area must be “at least” green, that is green or a color corresponding to a relatively higher number of waveform occurrences. As another example, the criteria might be an “average” color for all the pixels (bitmap cells) in the defined area. These are but examples—other color-based criteria may be employed. Furthermore, the specification occurring at 82 (FIG. 4) may be expressed directly through inputs from the user, or may be derived or calculated from user inputs.

Typically, as discussed above, the trigger test is performed once for every display frame sent to display subsystem 34. For example, in one of the previously-described embodiments, the system was updated with approximately 48,000 transforms every second, and a 33 Hz frame rate was employed. In this example, the trigger criteria would be tested for approximately once for every 1400 transforms applied to the bitmap.

Referring to the determination step 92 of FIG. 4, the determination process typically will involve a pixel-by-pixel check of the region of interest that is defined at step 80. If the color-based criteria are satisfied, triggering occurs and a capture analysis is taken, as shown at 94.

Triggering may also be predicated on waveform prevalence, also referred to as density or occupancy rate, with respect to the defined region of interest in the bitmap. Stated another way, a user may be interested in the relative rate at which a time-varying signal generates frequency spectrum components in a particular power-frequency region of the spectrum. Referring to FIG. 5, an exemplary frequency-power bitmap spectrum 100 is shown. Assume, for example, that the user expects a “healthy” signal to primarily or exclusively generate components in Region A, with occasional artifacts occurring in Region B. The user may be unconcerned with such artifacts, provided they occur infrequently. Accordingly, the user could define a prevalence or density threshold, which would result in a trigger (and analysis capture) in the event of increased activity in Region B. From this example, it should be understood that the present systems and methods can be used advantageously to detect spectral events even when they are “surrounded” on the spectral representation by signals of both higher and lower amplitudes, and by signals of greater and lesser prevalence. Furthermore, in many cases a useful trigger can be devised without any prior knowledge of the signal of interest's wave shape in either the time or frequency domain.

Density values may be determined in a variety of ways. In one implementation, density of a region is defined as the number of bitmap cell hits occurring in the region, divided by the number of hits that would have occurred if all the transforms from the respective observation period had fallen into the defined region. For example, assume the region of interest is defined at step 80 as being at frequency F, between power levels P_(upper) upper and P_(lower). Assume further that 100 spectrums are applied to the bitmap database during the observation interval. If 85 of the spectrums generate a power level between P_(upper) and P_(lower) for frequency F, then the density would be 85/100=0.85, using this measure. If the region of interest were to span a range of frequencies, for example so that it was i cells wide, then that factor would have to be accounted for in the denominator. The resulting calculation would involve dividing the number of occurrences in the region by (100×i) for an average density. Other equations could be employed for other definitions of density over an area, such as selecting the maximum or minimum density values among the i columns.

In addition to the above examples, triggering may be based on observing characteristics in the defined region of interest, and then determining the degree to which the observed characteristics correlate to a reference, such as a previously observed signal or signal event. Referring to FIGS. 6, 7 and 8, FIG. 6 shows a reference within a region of interest, while FIGS. 7 and 8 respectively show two different samples of collected spectrum data. Cursory inspection of the signal reveals that the sample of FIG. 7 more closely matches or correlates with the reference.

Various quantitative methods may be employed to assess the correlation of an observed bitmap region to a reference region. Generally, the bitmap region is updated, as in the example method of FIG. 4, and the correlation trigger is tested from time to time (e.g., once every display frame, or as often as after each transform). More particularly, during the testing step 92 of FIG. 4, a correlation score may be established for the collected data, where the score numerically reflects the degree to which the collected data correlates to the reference.

Referring again to FIG. 4, when correlation triggers are employed, the trigger definition at 82 may also include establishing or specifying a reference to be employed in the generation of correlation scores. In many cases, this will involve using previously obtained data, for example a reference DPX bitmap created from a portion of a time-varying signal known to be free of excessive noise or undesirable artifacts. On the other hand, the user might want to search for and diagnostically trigger upon a known spurious signal or artifact, in which case that would be supplied as the reference to the extent its characteristics were known.

The criteria-specifying in step 82 may also include the user specifying the amount of correlation between the observed signal and the reference that would trip the trigger. As previously indicated, correlation may be quantitatively measured in terms of a correlation score, and the trigger could thus be declared upon the correlation score exceeding a threshold value specified at step 82.

It will be appreciated that the reference for the correlation assessment may be obtained in a variety of ways. For example, the reference may be externally applied, in the sense that the reference is a predetermined signal phenomena obtained prior to the current data acquisition session. The external reference is supplied to the process for purposes of generating the trigger criteria. On the other hand, the reference may be internal, in the sense that it is generated or obtained from the transform and bitmap data being generated in the current data acquisition session. For example, the reference can simply be the accumulated (persisted) bitmap in the region of interest, so that a new signal that follows a path through the region of interest that has been followed many times before (and therefore has higher hit counts in the persisted display than a path less travelled) generates a trigger. In this case, there is no externally-supplied reference.

Furthermore, whether internal or external, the reference need not be taken from the region of interest that is specified for trigger testing. In some cases, a reference may be obtained from a different part of the bitmap than is being tested for the trigger criteria.

In connection with the various exemplary systems and methods described above, it will be appreciated that multiple copies and/or versions of the bitmap data may be employed, for the purposes of trigger testing, implementing persistence, driving display output, etc. FIG. 9 depicts various illustrative examples of the bitmap structures, as stored in memory subsystem 32. As shown, bitmaps may be retained and updated to reflect infinite persistence 120, zero persistence 122 (i.e., the data from the most current transform or display frame of transforms), and various implementations of variable persistence 124, 126, 128. Furthermore, any or all of these data structures may be used to drive the actual display bitmap 130 that is read out to display subsystem 34.

Triggering may also be performed based on hit counts occurring in the bitmap and/or in particular regions of interest on the bitmap. Hit count thresholds may be absolute counts, or relative to overall counts occurring in the bitmap, or counts occurring in particular regions of the bitmap. Furthermore, the density/occupancy and coloration triggers discussed herein may at times be expressed in terms of absolute or relative bitmap hit counts.

As previously indicated in connection with step 82, the user specification of trigger criteria may in some cases be derived from a user input, as opposed to being directly specified. For example, referring to FIG. 10, the figure depicts a rendering of a bitmap 140 as presented to user 142 on a touch-sensitive interactive display 144. Referring to both FIG. 10 and steps 80 and 82 of FIG. 4, the depicted example provides a user interface which allows the user to intuitively indicate both the region of interest and the signal phenomena that is to be used to supply the trigger definition 82. In particular, user 142 is shown as making selection 146 by making a touch input with reference to the display.

Based on selection 146, processing instructions may be implemented (for example, within processing subsystem 36 of FIG. 1) in order to generate the region of interest and triggering criteria based on the user selection 146. One advantage of this approach is that it is intuitive for the user, and allows the user to gesture to a signal event (for example, from a past saved spectral image) that they wish to serve as the basis for a trigger. Then, in response to the bitmap data within the selection 146, the processor automatically translates the user input into trigger requirements by analyzing spectral data in the designated region of the bitmap. These trigger requirements may be expressed or broken down into a variety of trigger types, including the color, density, hit count and correlation triggers described above, and/or combinations thereof. In some embodiments, it will be desirable to scale or otherwise implement proportionality in connection with the characteristics that are identified in the user selection 146. For example, if the system determines a value for the signal density or occupancy in the selected region, it will at times be desirable to scale down the value if used for a triggering threshold, so as to be slightly over-inclusive in the declaration of trigger events. This may be considered a “sensitivity” of the trigger, and the user interface or device settings typically will also include mechanisms to enable user adjustment of this sensitivity.

It will be appreciated that the embodiments described herein may be implemented, for example, via computer-executable instructions or code, such as programs, stored on a computer-readable storage medium and executed by a computing device. Generally, programs include routines, objects, components, data structures, and the like that perform particular tasks or implement particular abstract data types. As used herein, the term “program” may connote a single program or multiple programs acting in concert, and may be used to denote applications, services, or any other type or class of program. Likewise, the terms “computer” and “computing device” as used herein include any device that electronically executes one or more programs.

It will further be understood that the configurations and/or approaches described herein are exemplary in nature, and that these specific embodiments or examples are not to be considered in a limiting sense, because numerous variations are possible. The specific routines or methods described herein may represent one or more of any number of processing strategies. As such, various acts illustrated may be performed in the sequence illustrated, in other sequences, in parallel, or in some cases omitted. Likewise, the order of any of the above-described processes is not necessarily required to achieve the features and/or results of the embodiments described herein, but is provided for ease of illustration and description.

The subject matter of the present disclosure includes all novel and nonobvious combinations and subcombinations of the various processes, systems and configurations, and other features, functions, acts, and/or properties disclosed herein, as well as any and all equivalents thereof. 

1. A method for triggering upon signal events in an RF test and measurement device, comprising: repeatedly sampling a time-varying signal; generating, over time, a plurality of digital frequency domain spectrums based on the sampling of the time-varying signal; repeatedly updating a frequency domain bitmap for the time-varying signal by sequentially applying the digital frequency domain spectrums to a data store containing the frequency domain bitmap; and triggering a capture of the time-varying signal in response to detection of an occurrence of a pre-defined characteristic in the frequency domain bitmap.
 2. The method of claim 1, where the frequency domain bitmap is a two-dimensional grid with cells having specified frequency/power values for frequency and power, and where repeatedly updating the frequency domain bitmap includes, for each one of the digital frequency domain spectrums, incrementing specific cells of the frequency domain bitmap if corresponding values are present in said one of the digital frequency domain spectrums.
 3. The method of claim 2, further comprising periodically sampling the frequency domain bitmap and using said sampling to provide a spectrum display.
 4. The method of claim 3, where the spectrum display is configured so that coloration of individual cells in the spectrum display is dependent upon a prevalence of corresponding frequency/power values present in the digital frequency domain spectrums applied to the data store containing the frequency domain bitmap.
 5. The method of claim 4, where the predefined characteristic is expressed in terms of the coloration of individual cells on the spectrum display.
 6. The method of claim 5, where the spectrum display is configured so that intensity of individual cells in the spectrum display is dependent upon a prevalence of corresponding frequency/power values present in the digital frequency domain spectrums applied to the data store containing the frequency domain bitmap.
 7. The method of claim 2, where triggering a capture of the time-varying signal in response to detection of an occurrence of a predefined characteristic in the frequency domain bitmap includes determining, for a selected portion of the frequency domain bitmap, whether a spectrum waveform prevalence of the digital frequency domain spectrums exceeds a threshold in such selected portion.
 8. The method of claim 1, further comprising periodically sampling the frequency domain bitmap and using said sampling to provide a spectrum display on a display device.
 9. The method of claim 8, further comprising receiving a user input in reference to the spectrum display to establish the selected portion of the frequency domain bitmap.
 10. The method of claim 1, where triggering a capture of the time-varying signal includes: specifying a region of interest on the frequency domain bitmap; establishing a trigger criteria based on a color or intensity characteristic of a display device rendering of data from the region of interest; and performing the capture of the time-varying signal upon satisfaction of the trigger criteria.
 11. The method of claim 1, where triggering a capture of the time-varying signal includes specifying a region of interest on the frequency domain bitmap; establishing a trigger criteria based on a density characteristic of a display device rendering of data from the region of interest; and performing the capture of the time-varying signal upon satisfaction of the trigger criteria.
 12. The method of claim 1, where triggering a capture of the time-varying signal includes specifying a region of interest on the frequency domain bitmap; establishing a trigger criteria based on a bitmap hit count for the region of interest; and performing the capture of the time-varying signal upon satisfaction of the trigger criteria.
 13. The method of claim 1, where triggering a capture of the time-varying signal includes specifying a region of interest on the frequency domain bitmap; specifying a reference in the region of interest; establishing a trigger criteria based on a degree of correlation between collected spectral data and the reference; and performing the capture of the time-varying signal when collected spectral data meets or exceeds a threshold degree of correlation to the reference.
 14. An RF test and measurement device, comprising: a front end section for receiving a time-varying signal; a real-time engine operatively coupled with the front end section and configured to generate digital frequency domain spectrums based on the time-varying signal as the time-varying signal is received and processed through the front end section; and a memory subsystem containing a frequency domain bitmap which is updated through sequential receipt and storage of the digital frequency domain spectrums, where the real time engine is further configured to monitor the frequency domain bitmap for occurrence of a signal characteristic, and in response to detection of the signal characteristic, cause a capture of the time-varying signal into a storage location of the RF test and measurement device.
 15. The device of claim 14, further comprising a user interface configured to receive user inputs that yield specification of a region of interest on the frequency domain bitmap, and specification of the signal characteristic in terms of a color-based characteristic of the frequency domain bitmap, where the color-based characteristic pertains to coloration of the region of interest of the frequency domain bitmap when the frequency domain bitmap is rendered for display on a display device of the RF test and measurement device.
 16. The device of claim 14, further comprising a user interface configured to receive user inputs that yield specification of a region of interest on the frequency domain bitmap, and specification of the signal characteristic in terms of a density-based characteristic of the frequency domain bitmap, where the density-based characteristic is an occupancy rate within the region of interest of the frequency domain bitmap.
 17. The device of claim 14, further comprising a user interface configured to receive user inputs that yield specification of a region of interest on the frequency domain bitmap, and specification of the signal characteristic in terms of a degree of correlation between spectral data collected for the region of interest and a reference.
 18. The device of claim 17, where the reference is externally supplied.
 19. The device of claim 17, where the reference is generated based on internal data acquired from the frequency domain bitmap.
 20. A method for triggering upon signal events in an RF test and measurement device, comprising: establishing a region of interest on a frequency domain bitmap which is configured to store digitized power data as a function of frequency; specifying triggering criteria for the region of interest in terms of a predefined characteristic; repeatedly sampling a time-varying signal; generating, over time, a plurality of digital frequency domain spectrums based on the sampling of the time-varying signal; repeatedly updating the frequency domain bitmap by sequentially applying the digital frequency domain spectrums to a data store containing the frequency domain bitmap; and triggering a capture of the time-varying signal in response to detection of the pre-defined characteristic in the region of interest of the frequency domain bitmap.
 21. The method of claim 20, where specifying triggering criteria for the region of interest in terms of a predefined characteristic includes specifying coloration of the region of interest when the region of interest is rendered upon a display device of the RF test and measurement device.
 22. The method of claim 20, where specifying triggering criteria for the region of interest in terms of a predefined characteristic includes specifying an occupancy rate in the region of interest when the region of interest is rendered upon a display device of the RF test and measurement device.
 23. The method of claim 20, where specifying triggering criteria for the region of interest in terms of a predefined characteristic includes specifying a correlation threshold, where the correlation threshold indicates a degree of correlation existing between a reference and spectral data in the region of interest. 